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The long and short of 1.5oC

by Professor Richard Betts, University of Exeter and Met Office Hadley Centre

The Paris Agreement aims to limit global warming to well below 2°C above pre-industrial, and to ‘pursue efforts’ to stay below 1.5°C.  But last year, global temperature was more than 1°C above pre-industrial, and climate simulations indicate that the first individual year at 1.5°C could potentially occur in a decade’s time, depending on exactly when and how natural ups and downs such as El Nino add to the human-caused warming trend. Unless something enormous happens, such as a large volcano, worldwide collapse of modern society or deliberate large-scale geoengineering, ongoing warming is unavoidable for many more years.  So, what will happen when we first hit 1.5°C?

First off, it’s worth noting that the 2°C and 1.5°C ‘guardrails’ are generally considered to refer to long-term averages – a decade or more.   Anthropogenic climate change is generally viewed in terms of such long-term averages, to avoid confusion with the natural year-to-year variability of the climate system which temporarily adds and subtracts from the ongoing warming trend. So these global targets are not meant to refer to individual years.  Comparison with the 1°C record of 2015 is therefore not entirely consistent, as it was just one individual year.  The long-term average of recent years is still less than 1°C.

Having said that, it’s worth noting that Met Office climate models suggest that the global temperature will average above 1°C for the next five years.  We therefore could indeed be at the start of the ‘halfway mark’ to 2°C.  However, the definitions of 2°C and 1.5°C as key thresholds are somewhat approximate anyway.  Despite the apparently precise focus on specific temperatures, there is not really a well-known firm threshold for ‘dangerous’ climate change.  2°C is a convenient round number in Celcius, but translating into the equivalent warming of 3.6°F leads to artificial precision – nobody has suggested that a global climate threshold can be defined to within a tenth of a degree Fahrenheit.  Things will not suddenly switch when a particular level of warming is reached.  We are already seeing some impacts and risks of climate change, while others will take a very long time for their effects to be seen in full. 

Sea levels are already rising, and many species seem to be responding to warming. There is a general trend towards earlier springtime events such as flowers blooming, trees coming in to leaf, and migratory birds arriving from their winter grounds.  Calculations suggest that some kinds of extreme weather are becoming more likely – such as heatwaves and some heavy rainfall events. Crop yields may already be influenced by trends in climate and the rising CO2 concentration, although other factors such as technology and farming practices are currently still dominant.

When the impacts of climate change will become ‘dangerous’ – and indeed whether they already are - is something of a value judgement.  It is quite possible that many of the risks could be reduced by adaptation, at least in the short term, if the right plans and investments are made. Improving resilience, especially through poverty reduction in developing countries, will help global society adapt to the imminent changes that are already locked-in.  But it is unlikely that risks can be reduced to zero. 

More importantly, there are certain impacts that would clearly be severe, especially in the longer term.  If sea level rise is unchecked, this will ultimately swamp low-lying land areas and displace the people living there, unless there is investment in coastal defences.  Without such investment, sea level rise of half a metre could displace millions of people. And half a metre of sea level rise could occur within this century, even if global warming is limited to 2°C. 

Sea level rise could potentially be much larger in the longer term – up to a metre in a few hundred years, and 7m after thousands of years if the melting of the Greenland ice sheet passes the point of no return.  This point is hard to pin down, but it could well be around 2°C global warming.  

The potential for long-term sea level rise is one of the key reasons behind the ‘well below 2°C’ ambition.  Small Island Developing States, fearing that investment in coastal defences may be far too expensive or simply inadequate in the long term, regard inundation by the sea as an existential threat.   Several low-lying river deltas are highly populated, and many of the world’s major cities are also on the coast and vulnerable to rising seas. 

So, while we should not expect to see sudden changes in the first year at 1.5°C above pre-industrial, we should also understand the long-term risks.  We are exceedingly unlikely to see global climate catastrophe in the first 1.5°C year, or even subsequent years, so we should not despair and panic when this symbolic threshold is passed.  But equally, the absence of such catastrophe should not be used to claim that it’s all a fuss about nothing.  The first year at 1.5°C may not herald imminent climate doom, but it will be a reminder of what we are storing up for the future.